Modulation scheme for orthogonal frequency division multiplexing systems or the like

ABSTRACT

Briefly, in accordance with one embodiment of the invention, bit and power loading may be utilized to select a modulation rate and subcarrier power scaling based on channel state information. As a result, a higher data rate may be utilized for a given signal-to-noise ratio while maintaining a constant bit error rate.

BACKGROUND OF THE INVENTION

In typical orthogonal frequency division multiplexing (OFDM) systems,different modulation schemes may be utilized to provide different codingrates based on a signal-to-noise ratio experienced in a given channel.However, most such systems utilize coarse increments of throughput for agiven change in signal-to-noise ratio, typically 6 dB increments. Itwould be desirable to provide less coarse throughput increments withoutover a smaller change in signal-to-noise ratio without requiringadditional redundancy in the system so that a higher modulation rate maybe utilized for a given signal-to-noise ratio, while maintaining a fixedbit error rate for a higher number of subcarriers of each OFDM symbol.

DESCRIPTION OF THE DRAWING FIGURES

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a block diagram of a wireless local area network system inaccordance with one embodiment of the present invention; into.

FIG. 2 is a block diagram of an orthogonal frequency divisionmultiplexing transceiver in accordance with one embodiment of thepresent invention;

FIG. 3 is a block diagram of an encoder modulator in accordance with oneembodiment of the present invention; and

FIG. 4 is a block diagram of a multi-rate trellis coded modulationencoder in accordance with an embodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Some portions of the detailed description that follows are presented interms of algorithms and symbolic representations of operations on databits or binary digital signals within a computer memory. Thesealgorithmic descriptions and representations may be the techniques usedby those skilled in the data processing arts to convey the substance oftheir work to others skilled in the art.

An algorithm is here, and generally, considered to be a self-consistentsequence of acts or operations leading to a desired result. Theseinclude physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. It has proven convenient at times,principally for reasons of common usage, to refer to these signals asbits, values, elements, symbols, characters, terms, numbers or the like.It should be understood, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as processing, computing, calculating,determining, or the like, refer to the action or processes of acomputer, computing platform, or computing system, or similar electroniccomputing device, that manipulate or transform data represented asphysical, such as electronic, quantities within the registers ormemories of the computing system into other data similarly representedas physical quantities within the memories, registers or other suchinformation storage, transmission or display devices of the computingsystem.

Embodiments of the present invention may include apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the desired purposes, or it may comprise a generalpurpose computing device selectively activated or reconfigured by aprogram stored in the device. Such a program may be stored on a storagemedium, such as, but is not limited to, any type of disk includingfloppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-onlymemories (ROMs), random access memories (RAMs), electricallyprogrammable read-only memories (EPROMs), electrically erasable andprogrammable read only memories (EEPROMs), flash memory, magnetic oroptical cards, or any other type of media suitable for storingelectronic instructions, and capable of being coupled to a system busfor a computing device.

The processes and displays presented herein are not inherently relatedto any particular computing device or other apparatus. Various generalpurpose systems may be used with programs in accordance with theteachings herein, or it may prove convenient to construct a morespecialized apparatus to perform the desired method. The desiredstructure for a variety of these systems will appear from thedescription below. In addition, embodiments of the present invention arenot described with reference to any particular programming language. Itwill be appreciated that a variety of programming languages may be usedto implement the teachings of the invention as described herein.

In the following description and claims, the terms coupled andconnected, along with their derivatives, may be used. In particularembodiments, connected may be used to indicate that two or more elementsare in direct physical or electrical contact with each other. Coupledmay mean that two or more elements are in direct physical or electricalcontact. However, coupled may also mean that two or more elements maynot be in direct contact with each other, but yet may still cooperate orinteract with each other.

It should be understood that embodiments of the present invention may beused in a variety of applications. Although the present invention is notlimited in this respect, the circuits disclosed herein may be used inmany apparatuses such as in the transmitters and receivers of a radiosystem. Radio systems intended to be included within the scope of thepresent invention include, by way of example only, wireless local areanetworks (WLAN) devices and wireless wide area network (WWAN) devicesincluding wireless network interface devices and network interface cards(NICs), base stations, access points (APs), gateways, bridges, hubs,cellular radiotelephone communication systems, satellite communicationsystems, two-way radio communication systems, one-way pagers, two-waypagers, personal communication systems (PCS), personal computers (PCs),personal digital assistants (PDAs), and the like, although the scope ofthe invention is not limited in this respect.

Types of wireless communication systems intended to be within the scopeof the present invention include, although not limited to, WirelessLocal Area Network (WLAN), Wireless Wide Area Network (WWAN), CodeDivision Multiple Access (CDMA) cellular radiotelephone communicationsystems, Global System for Mobile Communications (GSM) cellularradiotelephone systems, North American Digital Cellular (NADC) cellularradiotelephone systems, Time Division Multiple Access (TDMA) systems,Extended-TDMA (E-TDMA) cellular radiotelephone systems, third generation(3G) systems like Wide-band CDMA (WCDMA), CDMA-2000, and the like,although the scope of the invention is not limited in this respect.

Referring now to FIG. 1, a wireless local area network communicationsystem in accordance with one embodiment of the present invention willbe discussed. In the WLAN communications system 100 shown in FIG. 1, amobile unit 110 may include a wireless transceiver 112 to couple to anantenna 118 and to a processor 114 to provide baseband and media accesscontrol (MAC) processing functions. Processor 114 in one embodiment maycomprise a single processor, or alternatively may comprise a basebandprocessor and an applications processor, although the scope of theinvention is not limited in this respect. Processor 114 may couple to amemory 116 which may include volatile memory such as DRAM, non-volatilememory such as flash memory, or alternatively may include other types ofstorage such as a hard disk drive, although the scope of the inventionis not limited in this respect. Some portion or all of memory 116 may beincluded on the same integrated circuit as processor 114, oralternatively some portion or all of memory 116 may be disposed on anintegrated circuit or other medium, for example a hard disk drive, thatis external to the integrated circuit of processor 114, although thescope of the invention is not limited in this respect.

Mobile unit 110 may communicate with access point 122 via wirelesscommunication link 132, where access point 122 may include at least oneantenna 120. In an alternative embodiment, access point 122 andoptionally mobile unit 110 may include two or more antennas, for exampleto provide a spatial division multiple access (SDMA) system or amultiple input, multiple output (MIMO) system, although the scope of theinvention is not limited in this respect. Access point 122 may couplewith network 130 so that mobile unit 110 may communicate with network130, including devices coupled to network 130, by communicating withaccess point 122 via wireless communication link 132. Network 130 mayinclude a public network such as a telephone network or the Internet, oralternatively network 130 may include a private network such as anintranet, or a combination of a public and a private network, althoughthe scope of the invention is not limited in this respect. Communicationbetween mobile unit 110 and access point 122 may be implemented via awireless local area network (WLAN), for example a network compliant witha an Institute of Electrical and Electronics Engineers (IEEE) standardsuch as IEEE 802.11a, IEEE 802.11b, HiperLAN-11, and so on, although thescope of the invention is not limited in this respect. In anotherembodiment, communication between mobile unit 110 and access point 122may be at least partially implemented via a cellular communicationnetwork compliant with a 3GPP standard, although the scope of theinvention is not limited in this respect.

Referring now to FIG. 2, a transceiver for an orthogonal frequencydivision multiplexing system in accordance with one embodiment of theinvention. The transceiver 200 of FIG. 2 may correspond, for example, tothe transceiver 112 of mobile unit 116 or to the transceiver 124 ofaccess point 122 of FIG. 1, although the scope of the invention is notlimited in this respect. The transceiver 200 shown in FIG. 2 may includea transmitter circuit 210 and a receiver circuit 228. In addition,transceiver 200 may include a bit and power loading circuit 244. Asshown in FIG. 2, binary data 212 is provided to a trellis codedmodulation (TCM) encoder 214 which may provide an output to aserial-to-parallel converter 216. The parallel data output ofserial-to-parallel converter 216 may be passed through a weighting block218 and then through an inverse fast Fourier transform (IFFT) block 220.The output of IFFT block 220 may then be passed through aparallel-to-serial converter block 222 where a cyclic prefix may beappended to the data in accordance with a orthogonal frequency divisionsystem, although the scope of the invention is not limited in thisrespect. The transmitter 210 may output OFDM data 224 to be transmittedto a remote device.

Receiver 228 may receive OFDM data 226 from a remote device which may beconverted from a series signal into a parallel signal viaserial-to-parallel converter block 230 where the cyclic prefix may beremoved from the received OFDM data 226. The parallel data fromserial-to-parallel converter block 230 may then be passed through a fastFourier transform (FFT) block 232, the output of which math then bepassed through an equalizer and weighting block 234. The output ofequalizer and weighting block 234 may be passed through aparallel-to-serial convert block 236, which may provide data to atrellis coded modulation (TCM) decoder 238. The decoded output providedby TCM decoder 238 is the desired binary data 240, although the scope ofthe invention is not limited in this respect.

In accordance with one embodiment of the invention, bit and powerloading block 244 may implement a bit and power loading algorithm (BPLA)based on received channel state information (CSI) 242 provided to theinput of bit and power loading block 244. In one embodiment of theinvention, the channel state information (CSI) may be obtained bytransceiver 200 from a remote device or a remote user. The remote usermay calculate channel state information by processing training symbolstransmitted by transceiver 210 during a previous packet transmission. Inone particular embodiment of the invention, channel state informationmay consist of a channel transfer function estimate in the frequencydomain or a channel response function estimate in the time domain. In analternative embodiment of the invention, a remote user may processchannel function estimates itself using a bit and power loading block,and may then transmit power allocation and modulation type instructionsas the ready to use channel state information back to the originaltransmitting device. Based at least in part on obtained CSI (ChannelState Information) 242, bit and power loading block 244 may determinewhich subcarriers, if any, that should be turned off, and may calculatethe power values and the rates, or signal constellations, for the activesubcarriers. Such information may be provided by bit and power loadingblock 244 to transmitter 210 and receiver 228 by providing powerallocation information 246 and 248 to equalizer and weighting block 234and to weighting block 218, and by providing modulation type information250 and 252 to TCM decoder 238 and TCM encoder 214 as shown in FIG. 2,although the scope of the invention is not limited in this respect.

In one embodiment of the invention, channel state information 242 may beavailable at the transmitter side. The transmitter side in oneembodiment may be defined as being a first device that transmits data toa remote device, where the remote device may transmit some channel stateinformation 242 back to the first device, although the scope of theinvention is not limited in this respect. For example, access point 122may transmit a signal, which may contain training symbols, to mobileunit 110, and then mobile unit may transmit the channel stateinformation 242 back to access point 122 so that transceiver 124 ofaccess point 122 may utilize the channel state information 242 inaccordance with the present invention, although the scope of theinvention is not limited in this respect. In such a case, access point122 may be considered as the transmitter side, and mobile unit 110 maybe considered the receiver side, although the scope of the invention isnot limited in this respect. In response to the channel stateinformation 242, transmitter 210 may turn off one or more badsubcarriers, where a bad subcarrier may be defined as a subcarrier ofthe OFDM signal having a lower gain, and may then divide the remainingactive, or turned on, subcarriers into one or more fixed subsets. In asubset, subset carriers may be appointed the same rate as a combinationof modulation and encoding at TCM encoder 214 and TCM decoder 238 andthen rescaled via weighting block 218 and equalizer and weighting block234 to provide weighted subcarrier powers, via bit and power loadingblock 244. In one embodiment of the invention, rescaling of subcarrierpowers may be performed by bit and power loading block 244 so as tomaintain a fixed bit error rate (BER) at the receiver side, for exampleat mobile unit 110 for the subcarriers in the subcarrier subsets. In aparticular embodiment, the bit and power loading scheme in combinationwith a trellis coded modulation scheme to provide a fixed bit error ratemay be optimized for an additive white Gaussian noise (AWGN) channel,and may thus mitigate an effect of channels having different frequencyselective fading, although the scope of the invention is not limited inthis respect.

Referring now to FIG. 3, a block diagram of a trellis coded modulationencoder in accordance with the present invention will be discussed.Based on the information obtained from bit and power loading block 244as shown in FIG. 2, TCM encoder 214 may subsequently extract from binarydata 212 a desired number of bits for mapping each active subcarrier,and then partitions a block of bits of binary data 212 into coded bits314 and uncoded bits 316. The coded bits 314 may be passed through aconvolutional encoder 312, whereas the uncoded bits 316 may be utilizedto determine a signal constellation point for corresponding activesubcarriers within the subset selected by convolutional encoder output.As shown in FIG. 3, a signal mapping block 310 may select theconstellation subset based at least in part on the output ofconvolutional encoder 312, and select the signal constellation pointbased at least in part on the uncoded bits 316, although the scope ofthe invention is not limited in this respect.

Referring now to FIG. 4, a block diagram of a trellis coded modulationencoder in accordance with one embodiment of the present invention willbe discussed, further showing details of a convolutional encoder andsignal mapping block. As shown in FIG. 4, convolutional encoder 312 inone embodiment may comprise a combination of bit time delays 410 andcombiners 412 that may receive the coded bits 314. In one particularembodiment of the invention, TCM encoder 214 may be a 64-state TCMencoder arranged to be optimized for an additive white Gaussian noise(AWGN) channel with quadrature amplitude modulations (QAM) of 16-QAM,32-QAM, 64-QAM, and 128-QAM. The signal mapping block 310 may selectfrom one of the available modulation types.

At the receiver block 228 as shown in FIG. 2, TCM decoder 238 may findthe allowed signal point sequence, which is closest in Euclidiandistance to the received sequence of signals. In one embodiment of theinvention, a Viterbi algorithm may be used to determine the closestsignal sequence as follows. At each trellis branch, receiver 228 maycompare the received signal with every signal allowed for that branch.The closest signal point may be saved in memory until final subsets aredetermined. The branch then may be labeled with the metric proportionalto the Euclidian distance between these two signal points. The Viterbialgorithm then may be applied to determine a maximum likelihood path inthe trellis to determine the subset sequence. After the subset sequenceis determined, the appropriate delayed subset elements, the storedclosest signal points, may be found and converted to output binary data240, although the scope of the invention is not limited in this respect.

Referring now to FIG. 5, a diagram of a throughput versessignal-to-noise ratio of a transceiver in accordance with one embodimentof the present invention will be discussed. In accordance with oneembodiment of the invention, throughputs for orthogonal frequencydivision multiplexing in megabits per second are shown on the verticalaxis and signal-to-noise ratio in decibels (dB) is shown on thehorizontal axis. The throughput for OFDM using transceiver 200 in whichtrellis coded modulation and bit and power loading is utilized is shownat 510, compared with standard convolutional coding, for example asutilized in the IEEE 802.11a standard, with code rate R=3/4 is shown at512. In one embodiment of the invention, both coding schemes may utilizethe same bit and power loading algorithm, although the scope of theinvention is not limited in this respect. As shown in FIG. 5, wheretransceiver 200 utilizes trellis coded modulation with bit and powerloading in accordance with the present invention, at SNR of 13 dB orgreater, a performance gain may be provided, with little or no loss inperformance for SNR less than 10 dB when using 16-QAM as a minimal ordermodulation for trellis coded modulation, although the scope of theinvention is not limited in this respect.

Although the invention has been described with a certain degree ofparticularity, it should be recognized that elements thereof may bealtered by persons skilled in the art without departing from the spiritand scope of the invention. It is believed that the modulation schemefor orthogonal frequency division multiplexing systems or the like ofthe present invention and many of its attendant advantages will beunderstood by the forgoing description, and it will be apparent thatvarious changes may be made in the form, construction and arrangement ofthe components thereof without departing from the scope and spirit ofthe invention or without sacrificing all of its material advantages, theform herein before described being merely an explanatory embodimentthereof, and further without providing substantial change thereto. It isthe intention of the claims to encompass and include such changes.

1. A method, comprising: receiving channel state information of acommunication channel; resealing subcarrier power of a signal based onthe channel state information; and adjusting a modulation rate based onthe channel state information.
 2. A method as claimed in claim 1,wherein said rescaling and said adjusting maintain a constant bit errorrate for at least one or more subcarriers of the signal.
 3. A method asclaimed in claim 1, wherein said rescaling includes turning offsubcarriers of the signal with lower gain values.
 4. A method as claimedin claim 1, wherein the modulation is trellis coded modulation.
 5. Amethod as claimed in claim 1, wherein said adjusting includes selectinga modulation for a subcarrier when a signal-to-noise ratio persubcarrier of the communication channel is greater than a predeterminedvalue, and selecting another modulation when the signal-to-noise ratioper subcarrier of the communication channel is less than a predeterminedvalue.
 6. An article comprising: a storage medium having stored thereoninstructions that, when executed by a computing platform, result insignal modulation adapted to a channel state by: receiving channel stateinformation of a communication channel; resealing subcarrier power of asignal based on the channel state information; and adjusting amodulation rate based on the channel state information.
 7. An article asclaimed in claim 6, wherein the instructions, when executed, furtherresult in signal modulation adapted to a channel state by maintaining aconstant bit error rate for at least one or more subcarriers of thesignal.
 8. An article as claimed in claim 6, wherein the instructions,when executed, further result in signal modulation adapted to a channelstate by turning off subcarriers of the signal with lower gain values.9. An article as claimed in claim 6, wherein the modulation is trelliscoded modulation.
 10. An article as claimed in claim 6, wherein theinstructions, when executed, further result in signal modulation adaptedto a channel state by selecting a modulation for a subcarrier when asignal-to-noise ratio per subcarrier of the communication channel isgreater than a predetermined value, and by selecting another modulationwhen the signal-to-noise ratio per subcarrier of the communicationchannel is less than a predetermined value.
 11. An apparatus,comprising: a modulation encoder to modulate a signal at a modulationrate based on channel state information of a communication channel; anda weighting block to rescale subcarrier power of the signal based on thechannel state information.
 12. An apparatus as claimed in claim 1,wherein said modulation encoder and said weighting block maintain aconstant bit error rate for at least one or more subcarriers of thesignal.
 13. An apparatus as claimed in claim 1, wherein said weightingblock turns off subcarriers of the signal with lower gain values.
 14. Anapparatus as claimed in claim 1, wherein the modulation encoder is atrellis coded modulation encoder.
 15. An apparatus as claimed in claim1, wherein said modulation encoder selects a modulation on a subcarrierwhen a signal-to-noise ratio per subcarrier of the communication channelis greater than a predetermined value, and selects another modulationwhen the signal-to-noise ratio per subcarrier of the communicationchannel is less than a predetermined value.
 16. An apparatus,comprising: an orthogonal frequency division multiplexing transceiver;and an omnidirectional antenna to couple to said orthogonal frequencydivision multiplexing transceiver; said orthogonal frequency divisionmultiplexing transceiver including a modulation encoder to modulate asignal at a modulation rate based on channel state information of acommunication channel, and a weighting block to rescale subcarrier powerof the signal based on the channel state information.
 17. An apparatusas claimed in claim 16, wherein said modulation encoder and saidweighting block maintain a constant bit error rate for at least one ormore subcarriers of the signal.
 18. An apparatus as claimed in claim 16,wherein said weighting block turns off subcarriers of the signal withlower gain values.
 19. An apparatus as claimed in claim 16, wherein themodulation encoder is a trellis coded modulation encoder.
 20. Anapparatus as claimed in claim 16, wherein said modulation encoderselects a modulation on a subcarrier when a signal-to-noise ratio persubcarrier of the communication channel is greater than a predeterminedvalue, and selects another modulation when the signal-to-noise ratio persubcarrier of the communication channel is less than a predeterminedvalue.